Various methods to form an image on a recording medium such as plain paper and overhead projection film have been realized, and are used for copiers, printers, and facsimile machines, for example. Especially, electrophotography is widely employed for image forming apparatuses, because it can form an image of high quality at high speed, but its cost is low.
When an image is formed using electrophotography, generally, a toner image that has not been fixed yet is formed on a recording medium. Then, a fixing unit applies heat and pressure to the toner image thereby to fix the toner image on the recording medium. A heat roller method is widely used for the application of the heat and the pressure because the heat roller method is of high speed and safe. In the heat roller method, the recording medium passes through a pressure unit (hereinafter, referred to as a nip unit) formed by a heated fuser roller and a pressure roller that opposes the fuser roller so as to apply pressure to the fuser roller, and is heated and pressed.
The fuser roller is heated by a heating member such as a halogen heater to a temperature of about 180° C. at which the fuser roller is operable. The fuser roller is mainly a metal roller made of iron or aluminum, for example. The heating of the metal roller up to the operable temperature may take a long time (hereinafter, referred to as a rise time), for example, several minutes due to the high heat capacity of the metal roller. To avoid this problem, even when the image forming apparatus is idle (hereinafter, referred to as a standby time), the fuser roller is heated and kept at a temperature slightly lower than the operable temperature.
However, even while being kept at a temperature not as high as the operating temperature, the image forming apparatus consumes power that is not directly needed for its operation. According to a study, the power consumption needed during this standby period equals 70-80 percent of the total energy consumption of the image forming apparatus.
Recent consciousness of the need of environmental protection has triggered many countries to legislate various power saving regulations. In Japan, the amended power saving law becomes more severe. In the United States, regulations such as energy star and Zero Energy Star Mode (ZESM) have been enforced. It is desired that, during the standby period, the power consumption of the image forming apparatus approaches zero in order to comply with such regulations.
If the power consumption during the standby period is made zero, the image forming apparatus would require several minutes to make itself immediately operable. Then, a user needs to wait for a long period until the image forming apparatus becomes immediately operable. The image forming apparatus becomes unuseful. To avoid this problem, the fuser roller needs to be heated up quickly. For example, the above ZESM requires the rise time be less than 10 seconds.
The temperature of the fuser roller can be quickly increased by increasing the amount of energy given per the unit of time. A method to increase the amount of energy given per the unit of time is to make a power rating high. In the case of domestic offices, a power supply is rated as 100 V-15 A. This means that the power rating cannot be raised above 1500 W. Actually, an image forming apparatus that consumes 200 V input voltage for high speed printing is available. The use of such a high speed printing machine usually requires special arrangement on the power supply system from which the 200 V power is supplied to the image forming apparatus. Another method to increase the amount of energy give per the unit of time is to use two 100V-15 A power supplies. However, this method is practical only if there are two separate power adaptors close to the image forming apparatus.
Yet another method is proposed in which a secondary power supply is used to increase the maximum amount of power at the rise time. The secondary power supply is typically a rechargeable secondary battery such as a lead-acid cell and a nickel cadmium cell. If repeatedly charged and recharged, such a rechargeable battery is gradually degraded. In addition, if a great current is discharged, the life of a rechargeable battery is shortened. The nickel cadmium cell, which has a relatively long service life even if a great current is repeatedly discharged, can charge and discharge only at most 500-1000 times. If the nickel cadmium cell is assumed to be repeatedly charged and discharged twenty times a day, the nickel cadmium cell has only a one month service life. Furthermore, since the rechargeable battery of high capacity requires some hours to be charged, it is not practical to use the rechargeable battery for applications that require repeated charging and discharging within a day. Accordingly, the rechargeable battery cannot realize a practical secondary power supply.
In a reference document 1, a technique is disclosed in which a capacitor of great capacity such as an electric double layer capacitor is used as a second power supply. The electric double layer capacitor has two advantages. The first advantage is that the electric double layer capacitor can repeatedly charge and discharge more than several tens of thousands times. This means that there is substantially no limitation. Since the charging properties of the electric double layer capacitor degrade little, the electric double layer does not need to be replaced periodically. The second advantage is that the electric double layer capacitor can be charged for a short time period such as several to several tens of seconds. In addition, the electric double layer capacitor can flow a great current of several tens through several hundreds amperes. The electric double layer capacitor can be made ready to supply power.
That is, the electric double layer capacitor as a secondary power supply can supply power more than the limitation of the commercial power supply during a short rise time of several to several tens of seconds. Accordingly, a fixing unit of short rise time, high reliability, and high durability is realized.
A mechanism to take out great power is needed in order to exhaust the power stored in the capacitor of high capacity in the rise time of several to several tens of seconds. Since power=voltage×current, either the voltage or the current may to be increased.
The maximum current of a halogen heater that is used for heating the fuser roller is about 10-12 A. If a greater current is supplied, the life of the halogen heater is shortened. It is difficult to increase the maximum current. Accordingly, the power supplied to the halogen heater needs to be increased by making voltage high.
However, in the case in which water is used as the electrolyte, the voltage that a high-capacity capacitor cell can output is about 1 V, and even in the case in which organic solvent is used as the electrolyte, the voltage that a high-capacity capacitor cell can output is several volts. The voltage is kept low for preventing the electrolyte from being decomposited by electrolysis. If a great voltage needs to be supplied to the halogen heater, ten through several tens of cells need to be connected in series. In this case, even if a certain number of cells are enough to provide the halogen heater with sufficient energy, additional cells that are excessive energy-wise also need to be provided in order to increase the output voltage. This increases the cost of the power supply, and excessively increase the volume thereof.
To solve this problem, a reference document 2 proposes a method of obtaining great power at a low voltage by connecting a halogen heater to the high-capacity capacitor in parallel. According to this method, taking advantage of the fact that the high-capacity capacitor can flow a great current, great power is taken out at a low voltage in a short time period by setting the resistance of the halogen heater low. Thus, the cost of the capacitor power supply can be lowered, and the power supply can be made compact.    (Reference document 1) Japanese Laid-Open Patent Application No. 2000-315567    (Reference document 2). Japanese Laid-Open Patent Application No. 2000-184554
However, if the halogen heater is connected to the high-capacity capacitor in parallel, and a great current is flowed, wiring may cause a great loss as Joule heat. Since a great current flows through the wire, the wire needs to be very thick in order to reduce the loss as Joule heat. However, thick wire makes wiring in the image forming apparatus very difficult. The thick wire requires large space, and makes assembly difficult. On the other hand, if the wire is made short to reduce its resistance, the wire can be made relatively thin. As a result, wiring becomes easy, and it is possible to reduce space for wiring and to make assembly easy.
In the case of an electrophotography type image forming apparatus, the internal temperature often rises to 70-80° C. due to heat generated by the fixing unit and heat generated by the recent increase in power consumption of electric circuits. If the wiring is made short, the high-capacity capacitor and the fixing unit are disposed relatively close to each other. The environmental temperature of the high-capacity capacitor is increased due to the heat generation of the fixing unit.
In general, high-capacity capacitors such as electric double layer capacitors and pseudo capacitors called electro-chemical capacitors have temperature ranges suitable for operation. For example, the operating temperature of a gold capacitor manufactured by Matsushita Electric Industries KK is about −25° C.-+60° C. The operating temperature of Lithium Ion Batteries and Nickel Hydrogen Batteries of the same manufacturer is, in the case of charging, 0° C.-+45° C., and in the case of discharging, −10° C.-+60° C. In addition, the durability of the high-capacity capacitors is degraded at an excessively high temperature. This degradation is thought to be caused mainly by the degradation of the internal electrolyte. Since the degradation of the durability of the electric double layer capacitors due to temperature increase is caused by chemical reactions, it is thought that, if the environmental temperature rises by 10° C., the speed of degradation becomes twice based on Arrhenius's equation.
If the wire is made short in order to reduce heat generation and power loss caused by a great current, the high-capacity capacitor and the fixing unit need to be disposed relatively close to each other. The high-capacity capacitor may be degraded due to the heat generated by the fixing unit, and as a result, the durability of the image forming apparatus may be degraded.
In addition, the fixing unit is rapidly heated in about 10 seconds by the secondary power supply using the high-capacity capacitor. If image forming is performed immediately after the fixing unit being made operable, little heat generated for the fixing unit has been transferred to the external cover or components near a fixation part of the fixing unit. They are almost at the same temperature. Moisture from paper passing through the fixation part is condensed by the components that are at the same temperature. The condensed moisture, if it forms water droplet and enters the circuit of the secondary power supply, may cause a short circuit or leak. In the case in which a non-water electric double layer capacitor using organic solvent as electrolyte is used as a charging element, if the water enters the capacitor cells through a relief valve or a junction part, the performance is considerably degraded. Since the secondary batteries use organic solvent, the entering of water into the secondary batteries cause considerable degrading of performance.